Japanese Patent Application Kokai No. Hei 8-6175 discloses an illumination optical apparatus suited to exposure apparatus for manufacturing semiconductor devices. This illumination optical apparatus uses an internal reflection, elongate (i.e., rod-type) optical integrator to form a plurality of light source images from a primary light beam from a light source. Such integrators are referred to in the art as "light pipes" and so this term is used hereinafter.
The above-mentioned prior art illumination optical apparatus includes a condenser lens which condenses the primary light beam onto the incident surface of the integrator. This beam is then split by internal reflection within the light pipe into a plurality of secondary light beams. These secondary light beams proceed in predetermined angular directions based on the geometry of the integrator. A plurality of light source images (i.e., virtual light sources) associated with the plurality of secondary light beams are formed along the plane of the incident surface of the integrator. The plurality of secondary light beams, each appearing to emanate from a corresponding light source image, pass through a condenser lens and illuminate a surface to be irradiated, such as a mask.
The increasing degree of integration of semiconductor devices has lead to the commercialization and development of excimer lasers and other intense light sources for use in exposure apparatus for manufacturing semiconductor devices. Excimer lasers, for example, operate at an oscillation wavelength of 248 nm or 193 nm and have a high power output. Accordingly, conventional illumination optical apparatus, such as discussed immediately above, are not generally amenable for use in an exposure apparatus employing such a high-output light source. This is because a light pipe formed of glass material is prone to breaking due to the concentration of light energy at a convergence point formed on the incident surface of the integrator.